The present invention relates to a comb filter, the frequency characteristics of which have the appearance of a comb and which is used for various types of processing such as color separation of an image signal.
FIG. 14 is a block diagram illustrating an exemplary configuration of a conventional comb filter. In the comb filter shown in FIG. 14, an input image signal SI is supplied to a 1H delay line 101 for delaying the signal by 1H (i.e., one horizontal retrace interval) and to an adder 102. The output of the 1H delay line 101 is amplified by an AGC (automatic gain control) amplifier 103 and then added to the input image signal SI by the adder 102. The sum is output as an output signal SO having comb-like frequency characteristics. The gain of the AGC amplifier 103 is controlled by a gain controller 104 such that the variation in the gain of the 1H delay line 101 is eliminated (see, for example, Japanese Laid-Open Publication No. 61-144997).
Comb filters having similar configurations are also disclosed in Japanese Laid-Open Publications Nos. 4-114591 and 4-365291.
In a conventional comb filter, a burst signal is used as a control signal for controlling the gain of a delay circuit.
FIG. 15 is a diagram illustrating a burst signal. A "burst signal" is a reference signal to be inserted one by one into an image signal such a TV signal or a video signal every horizontal retrace interval. The burst signal has a predetermined frequency and the amplitude thereof is constant. In the comb filter shown in FIG. 14, while a burst interval detector 105 is detecting the input of the burst signal, the gain controller 104 is operating to control the gain of the AGC amplifier 103.
However, the signal level of a burst signal is sometimes varied or dropped. For example, in a TV signal, if the output transmitted varies, then the signal level of the burst signal also varies. During the reproduction of a video signal, the burst signal may be sometimes dropped depending on the recording states of a video tape. In actuality, if a dropout superimposes with a burst signal when a signal is reproduced by a video tape recorder, for example, the frequency characteristics of a comb filter cannot be kept normal.
Thus, a conventional comb filter is provided with a capacitor for storing burst signals such that the frequency characteristics thereof are not adversely affected by the variation or dropping of the burst signals. A considerable number of burst signals (on the order of 100 to 200) are stored in the capacitor and used for controlling the gain. Such a capacitor has large capacitance (ordinarily 1 .mu.F or more) and cannot be built in an LSI constituting a comb filter. Thus, such a capacitor is provided as an external component for the LSI.
The conventional comb filter uses a CCD, enabling efficient transfer of charges, as a delay device. However, since a CCD is fabricated through a different process from that applied to the other circuit components, the CCD cannot be incorporated into the LSI constituting a comb filter, either. Accordingly, a CCD also needs to be provided as an external component for the LSI.
As can be understood, since a large-capacitance capacitor and a CCD should be provided as external components for an LSI in accordance with the prior art, it is very difficult to implement a conventional comb filter as a monolithic LSI.